A technique of implementing an optical switch using a micromirror has been proposed (T. Yamamoto, et al., “A three-dimensional MEMS optical switching module having 100 input and 100 output ports”, Photonics Technology Letters, IEEE, Volume 15, Issue: 10). FIG. 37 shows a conventional optical switch using a micromirror.
The optical switch shown in FIG. 37 includes input ports 1a, output ports 1b, input-side micromirror array 2a, and output-side micromirror array 2b. Each of the input ports 1a and output ports 1b includes a plurality of optical fibers arrayed two-dimensionally. Each of the micromirror arrays 2a and 2b includes a plurality of micromirror devices 3a and 3b arrayed two-dimensionally. The arrows in FIG. 37 indicate a light beam traveling direction.
An optical signal which has outgone from a given input port 1a is reflected by the mirror of a micromirror device 3a of the input-side micromirror array 2a corresponding to the input port 1a so that the traveling direction changes. As will be described later, the mirror of the micromirror device 3a is designed to pivot about two axes so as to direct light reflected by the micromirror device 3a to an arbitrary micromirror device 3b of the output-side micromirror array 2b. The mirror of the micromirror device 3b is also designed to pivot about two axes so as to direct light reflected by the micromirror device 3b to an arbitrary output port 1b by appropriately controlling the tilt angle of the mirror. It is therefore possible to switch the optical path and connect arbitrary two of the input ports 1a and output ports 1b arrayed two-dimensionally by appropriately controlling the tilt angles of mirrors in the input-side micromirror array 2a and output-side micromirror array 2b. 
The most characteristic constituent elements of the optical switch are the micromirror devices 3a and 3b each having a mirror. In a micromirror device, conventionally, a mirror substrate 200 having a mirror and an electrode substrate 300 having electrodes are arranged in parallel, as shown in FIGS. 38 and 39 (see the above-described reference).
The mirror substrate 200 includes a plate-shaped frame portion 210, a gimbal 220 arranged in the opening of the frame portion 210, and a mirror 230 arranged in the opening of the gimbal 220. The frame portion 210, torsion springs 211a, 211b, 221a, and 221b, the gimbal 220, and the mirror 230 are integrally formed from, e.g., single-crystal silicon. For example, a Ti/Pt/Au layer having a three layer structure is formed on the surface of the mirror 230. The pair of torsion springs 211a and 211b connect the frame portion 210 to the gimbal 220. The gimbal 220 can pivot about a gimbal pivot axis X in FIG. 38 which passes through the pair of torsion springs 211a and 211b. Similarly, the pair of torsion springs 221a and 221b connect the frame portion 230 to the gimbal 220. The mirror 230 can pivot about a mirror pivot axis Y in FIG. 38 which passes through the pair of torsion springs 221a and 221b. The gimbal pivot axis X and the mirror pivot axis Y are perpendicular to each other. As a result, the mirror 230 pivots about the two axes which are perpendicular to each other.
The electrode substrate 300 includes a plate-shaped base portion 310, and a terrace-shaped projecting portion 320. The base portion 310 and the projecting portion 320 are made of, e.g., single-crystal silicon. The projecting portion 320 includes a second terrace 322 having a truncated pyramidal shape and formed on the upper surface of the base portion 310, a first terrace 321 having a truncated pyramidal shape and formed on the upper surface of the second terrace 322, and a pivot 330 having a columnar shape and formed on the upper surface of the first terrace 321. Four electrodes 340a to 340d are formed on the four corners of the projecting portion 320 and the upper surface of the base portion 310 led out of the four corners. A pair of projecting portions 360a and 360b are formed on the upper surface of the base portion 310 to be juxtaposed while sandwiching the projecting portion 320. Interconnections 370 are formed on the upper surface of the base portion 310. The electrodes 340a to 340d are connected to the interconnections 370 via leads 341a to 341d. An insulating layer 311 made of, e.g., silicon oxide is formed on the surface of the base portion 310. The electrodes 340a to 340d, leads 341a to 341d, and interconnections 370 are formed on the insulating layer 311.
The lower surface of the frame portion 210 and the upper surfaces of the projecting portions 360a and 360b are bonded to each other to make the mirror 230 face the electrodes 340a to 340d so that the mirror substrate 200 and the electrode substrate 300 form a micromirror device shown in FIG. 39. In the micromirror device, the mirror 230 is grounded. A positive driving voltage is applied to the electrodes 340a to 340d such that an asymmetrical potential difference is generated between them, thereby attracting the mirror 230 by an electrostatic attraction and making it pivot in an arbitrary direction.